The present invention relates to an electrophotographic imaging member having an improved charge blocking layer.
Typical electrophotographic imaging members comprise a photoconductive layer comprising a single layer or composite layers. One type of composite photoconductive layer used in xerography is illustrated, for example, in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. The disclosure of this patent is incorporated herein in its entirety. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer the photogenerating layer sandwiched between the contiguous charge transport layer and the supporting conductive layer, the outer surface of the charge transport layer is normally charged with a uniform charge of a negative polarity and the supporting conductive layer is utilized as an anode. The supporting conductive layer may also function as an anode when the charge transport layer is sandwiched between the supporting conductive layer and a photgenerating layer. The charge transport layer in this latter embodiment must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
As more advanced, complex, highly sophisticated, electrophotographic copiers, duplicators and printers were developed, greater demands were placed on the photoreceptor to meet stringent requirements for the production of high quality images. For example, the numerous layers found in many modern photoconductive imaging members must be uniform, free of defects, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a drum or belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a charge blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. This photoreceptor may also comprise additional layers such as an overcoating layer. Although excellent toner images may be obtained with multilayered photoreceptors, it has been found that the numerous layers limit the versatility of the multilayered photoreceptor. For example, these photoreceptors often comprise a metal substrate having a roughened surface to avoid plywooding effects that can occur with laser exposure systems. It has been found that when drum substrates are dip coated, the charge blocking layer does not consistently form a thick uniform coating on the roughened surface and often leaves uncovered bare spots at the peaks of the toughened substrate surface. These bare spots directly impact copy print quality because they print out as white spot defects on negatively charged photoreceptors. Also, the charge blocking layer coating tends to spontaneously develop extensive layer cracking after drying at elevated temperatures used to facilitate curing. Cracks developed in charge blocking layers during cycling are manifested as print-out defects which adversely affected copy quality. Moreover, alteration of materials in the various photoreceptor layers such as the charge blocking layer can adversely affect overall electrical, mechanical and other electrophotographic imaging properties such as residual voltage, background, dark decay, adhesion and the like, particularly when cycled thousands or hundreds of thousands of times in environments where conditions such as humidity and temperature can change daily. Thus, there is a great need for mass produced dip coated photoreceptors exhibiting high quality and long service life.
In a flexible seamed electrophotographic imaging belt configuration, good adhesion bond strength at all the multilayered contacting interfaces is of crucial importance to assure physical/mechanical integrity of the imaging member belt as well as elimination of seam delamination problems which frequently develop due to the result of repeated fatigue belt cycling over small diameter belt support rollers and poor adhesion bond strength at the contacting interfaces of the charge blocking layer during image cycling. The application of a silane hole blocking layer in a typical flexible electrophotographic imaging member web by a solution coating process can lead to an inherent physical shortfall where a non-uniform coating layer thickness is formed due to the presence of islands of siloxane aggregates. The existance of these siloxane aggregates in the charge blocking layer has been determined to be as one of the major drivers which cause the development of charge deficient spots observed as defects in final print copies.
There are numerous applications in the electrophotographic art wherein a coherent beam of radiation, typically from a helium-neon or diode laser, is modulated by an image data input signal. The modulated beam is directed (scanned) across the surface of a photosensitive medium. The medium can be, for example, a photoreceptor drum or belt in a xerographic printer. Certain classes of photosensitive medium can be characterized as "layered photoreceptors" having at least a partially transparent photosensitive layer overlying a conductive ground plane. A problem inherent in using these layered photoreceptors, depending upon their physical characteristics, is an interference effect created by two dominant reflections of the incident coherent light on the surface of the photoreceptor; e.g., a first reflection from the top surface and a second reflection from the bottom surface of the relatively opaque conductive ground plane. Spatial exposure variations present in the image formed on the photoreceptor become manifest in the output copy derived from the exposed photoreceptor. The output copy exhibits a pattern of light and dark interference fringes which look like the grains on a sheet of plywood, hence the expression "plywood effect" is generically applied to this problem. This phenomenon will be described in greater detail hereinafter.